1. Field of the Invention
This invention relates to casting molten steel.
2. Description of the Prior Art
In normal practice, molten steel produced by any of the classic processes, for example, the B.O.F., the Q.B.O.P., or the electric furnace process, usually contains a high level of oxygen. This degrades the steel. To overcome this, the steel is killed by introducing into the molten steel deoxidizing agents, for instance, silicon, in the form of ferro silicon or aluminum or both. This is usually performed in a transfer ladle, at tap.
When Al is used the steel is referred to as Al-killed and when Si is used the steel is referred to as Si-killed steel. The non-metallic impurities intentionally formed are allowed to decant and leave the body of molten steel, to be collected at the less dense slag layer floating over the steel.
Following deoxidation treatment, the killed molten steel has a strong affinity for oxygen, which it picks up when exposed to the atmosphere, during pouring from a furnace, or casting into ingot molds, into billets, or into slabs. This results in defects, for example, non-metallic inclusions, in the resulting steel which can reduce the quality of the finished products. For example, inclusions are formed by reaction of elements normally present in steel in concentrations of less than 2%, such as Ca, Mg, Al, Mn, B, Ti, P, Si, Cr, S, with either oxygen or nitrogen. The former are referred to as oxides and the latter as nitrides. When molten steel is exposed to air, formation of both oxides and nitrides can occur.
So, during casting operations and during molten steel transfer stages nearing solidification, new inclusions can be formed if surrounding oxygen or nitrogen is allowed to react with the aforementioned metallic elements. These inclusions, which can be as small as 1 micron and as large as 1000 microns, do not have enough time to float to the surface and, therefore, stay in the body of the solidified steel as non-desirable inclusions.
To prevent or reduce this, various protective methods have been used. One involves shielding open cast steel streams between tundish and mold with ceramic tubes. This has been established practice for maintaining high quality in continuous casting of large bloom and slab sections. It cannot be applied to smaller bloom and billet sections, however, because of space limitations. An example of this type of process is found in Canadian Pat. No. 1,097,881, Thalmann et al, Mar. 24, 1981.
Inert gases such as argon and helium are also well known agents used to protect the molten metal stream or surface during transfer operations. These gases are relatively scarce and, therefore, expensive. Nitrogen gas is presently used when the nitride content is not a critical specification of the finished steel product. More specific expedients are described as follows. The inert gas shrouding of strand cast steel has also been described in the article "Gas Shrouding of Strand Cast Steel at Jones & Laughlin Steel Corporation" by Samways, Pollard & Fedenco, Journal of Metals, October 1974. U.S. patents relating to this method are U.S. Pat. No. 3,908,734, Sept. 30, 1975, U.S. Pat. No. 3,963,224, June 15, 1976, and U.S. Pat. No. 4,023,614, May 17, 1977, all to Pollard.
Another method uses liquid nitrogen to form a shroud about the molten steel as it is teemed into a continuous casting machine. This is described in the brochure entitled "Conspal Surface Protection", published by Concast AG, Zurich, Switzerland, March 1977 and in U.S. Pat. No. 4,178,980 (1979), L'Air Liquide. In general, liquid nitrogen has provided a degree of protection which gives some improvement over other methods. But, handling this substance under the hard conditions of the pouring floor makes it difficult to provide continuity of flow, during the operation. Also, nitrogen has a density close to that of air, reducing its ability to displace air effectively. Moreover, nitrogen inerting is not practicable for grades of steel where nitride formation is undesirable.
The disclosures of the publications and patents mentioned are hereby incorporated by reference.